Question

Calculati suma primilor trei termeni ai progresiei aritmetice (an)n≥1 stiind ca a1=6 si a2=12.

Answer 1

6+12+18=36....adica a1+a2+a3

Answer 2

$\displaystyle a_1=6,~a_2=12 \\ \\ a_1=a_2-r \\ \\ a_3=a_2+r \\ \\ a_1+a_2+a_3=a_2-r+a_2+a_2+r=3a_2=3 \cdot 12=\underline{\underline{36}}$